Hydraulic fluids are low viscosity fluids used for the transmission of useful power by the flow of the fluid under pressure from a power source to a load. A liquid hydraulic fluid generally transmits power by virtue of its displacement under a state of stress. Hydraulic fluids generally operate with a low coefficient of friction. To be effective, the compositions typically have sufficient antiwear, antiweld, and extreme pressure properties to minimize metal damage from metal-to-metal contact under high load conditions.
Hydraulic fluids are usable in subsea control devices that are used to control well-head pressure of an oil well under production. The hydraulic equipment can open or close a well, choke the oil or gas flow, inject chemicals into the well or divert water and/or gas into the well to re-pressurise the system. Some of the hydraulic components are placed within the well, such as the Down Hole Safety Valve and ‘Smart Well’ flow control systems.
One of the biggest challenges in the oil and gas industry is to ‘produce’ oil and gas from harsher environments with high pressure and temperature. Since part of the hydraulic system is within the well, the hydraulic equipment and the associated fluid must also be suitable to survive these temperatures and maintain performance. In addition, the demand for aqueous based hydraulic fluid compositions such as may be used in subsea devices continues to increase due to the environmental, economic and safety (e.g. non-flammability) advantages of such fluids over conventional non-aqueous, oil-type hydraulic fluids.
Many conventional hydraulic fluids are not suitable for marine and deep sea applications due to their low tolerance to sea water contamination or to contamination by hydrocarbons, i.e., they tend to readily form emulsions with small amounts of seawater. Furthermore, in marine environments, problems arise due to the lack of biodegradability of the hydraulic fluid and to bacterial infestations arising in the hydraulic fluid, especially from anaerobic bacteria such as the sulphate reducing bacteria prevalent in sea water.
Other problems associated with the use of conventional hydraulic fluids under the extreme conditions encountered in marine and deep sea devices include: (1) some conventional hydraulic fluids may cause corrosion of metals in contact with the fluid; (2) some conventional hydraulic fluids are reactive with paints or other metal coatings or tend to react with elastomeric substances or at least cause swelling of elastomeric substances; (3) poor long-term stability, especially at elevated temperatures; (4) some hydraulic fluids require anti-oxidants to avoid the oxidation of contained components; (5) some hydraulic fluids are not readily concentrated for ease in shipping; and (6) may conventional hydraulic fluids have a non-neutral pH, thereby enhancing the opportunity for reaction with materials in contact with it. For all of these reasons, it has become advantageous to use aqueous hydraulic fluids in certain marine and deep sea applications and various aqueous formulations have been developed that are usable in such applications.
The OSPAR Convention for the Protection of the Marine Environment of the North-East Atlantic provides a framework for environmental requirements of chemicals used offshore. There are currently few if any water based fluids that can maintain lubrication at high temperature and meet the required environmental profile.
The inventor of the present invention has identified other lubricants that provide good lubricity and good stability for use under the extreme conditions encountered in subsea devices. In particular the inventor of the present invention has determined that salts of a diacid can be used with good results to improve lubricity of an aqueous hydraulic fluid composition.